A multiple input multiple output (MIMO) scheme is used to increase system capability in such a manner that a base station (BS) and/or a user equipment (UE) spatially transmit several data streams simultaneously by using two or more transmit (Tx) antennas. Transmit diversity enables data transmission with high reliability in a fast time-variant channel by transmitting the same stream through a plurality of Tx antennas. Spatial multiplexing increases system capability by transmitting different data streams through a plurality of Tx antennas.
Spatial multiplexing for a single user is called single user-MIMO (SU-MIMO). Channel capacity of the MIMO system increases in proportion to a minimum value between the number of Tx antennas and the number of receive (Rx) antennas. Spatial multiplexing for multiple users is called spatial division multiple access (SDMA) or multi user-MIMO (MU-MIMO).
A single codeword (SCW) mechanism and a multiple codeword (MCW) mechanism can be used in the spatial multiplexing. The SCW mechanism transmits N (N>1) data streams, which are transmitted simultaneously, by using one codeword. The MCW mechanism transmits N data streams by using M (M≦N) codewords. Each codeword is generated by separate channel encoding so that error detection can be performed separately.
Long-term evolution (LTE) is currently under study in the 3rd generation partnership project (3GPP) and is one of the latest standards of mobile communication techniques. Wireless access of the LTE is called an evolved-UMTS terrestrial radio access network (E-UTRAN). The LTE supports multiple antennas, and can support both SU-MIMO and MU-MIMO. Further, the LTE supports hybrid automatic repeat request (HARQ) in both uplink transmission and downlink transmission. Downlink HARQ implies that, when the BS transmits downlink data, the UE transmits an ACK/NACK signal for the downlink data. Uplink HARQ implies that, when the UE transmits uplink data, the BS transmits an ACK/NACK signal for the uplink data.
To transmit the ACK/NACK signal for the uplink data, the LTE system defines a physical hybrid-ARQ indicator channel (PHICH). According to a current LTE system, the uplink HARQ uses only the SCW mechanism and does not support the MCW mechanism. Therefore, the PHICH is designed such that only one ACK/NACK signal is provided for one uplink data (i.e., codeword). This implies that the uplink HARQ cannot support SU-MIMO according to the existing LTE system.
As uplink transmission requires a higher transmission capacity and a higher data rate and also as the UE can use a more number of Tx antennas, it becomes necessary for uplink transmission to support SU-MIMO. A method of transmitting or receiving an acknowledgment (ACK)/not-acknowledgment (NACK) signal for multiple codewords needs to be taken into account in order to perform uplink HARQ in SU-MIMO.